1. Field of the Invention
The present invention relates to a dry-type developing device capable of converting an electrostatic latent image formed on the surface of a latent image carrying member to a visible image by use of developer powder.
2. Description of the Related Art
In the image forming device such as a copying machine, facsimile device, printer, etc., a dry-type developing device is widely adopted as a type of image forming device which forms electrostatic latent image on a latent image carrying body thereof and converts the latent image to a visible image.
In such a developing device as mentioned above, first and second developer carrying rollers are provided, and the first developer carrying roller carries and transports therearound two-components developer consisting of nonmagnetic toner and magnetic carrier as a magnetic brush. Toner contained in the developer constructing the magnetic brush is electrostatically sucked onto the second developer carrying roller. And further, the sucked toner on the second developer carrying roller is transported to the side of the latent image carrying body such as a photosensitive body, and thereafter the electrostatic latent image formed on the latent image carrying body is developed as a visible image. Such technology mentioned above is well known. For instance, it is disclosed in Japanese Laid-open Patent Publication No. 60-140277/1985.
In addition to the above-mentioned, there exist several prior-art documents relevant to the above technology. Both of Japanese Laid-open Patent Publications Nos. 62-279374/1987 and 62-279375/1987 disclose an improvement technology of an electrophotographic device having a first roll for developing electrostatic latent image and a second roll for supplying the developer to the first roll.
Japanese Laid-open Patent Publication No. 56-40862/1981 relates to an electrophotographic developing device by use of one-component developer capable of coating uniformly the surface of the developing roller with toner, improving the clearness of the developed image without any developing uniformity, and obtaining the visible image having sufficient halftone characteristic.
On the other hand, Japanese Laid-open Patent Publication No. 56-126870/1981 relates to an improvement of the so-called jumping developing device in which one-component magnetic developer is applied onto the surface of the developer carrier in a state of thin layer and the thin layer surface of the developer is put adjacent to the surface of the member to be developed having electrostatic latent image so that the developer flies (jumps) toward the surface of the member to be developed from the developer thin layer by the action of electric force exerted by the latent image.
In the developing device as mentioned, for instance, in the Japanese Laid-open Patent Publication No. 60-140277/1985, when the magnetic brush formed on the first developer carrying roller frictionally slides on the surface of the second developer carrying roller, toner contained in the developer for constructing the magnetic brush is electrostatically attached to the second developer carrying roller.
However, on this occasion, a streaked portion with the thinning thickness of the toner layer which is the "so-called brush mark" is created on the second developer carrying roller by frictionally sliding on the surface of the second developer carrying roller.
Consequently, it turns out to be difficult to make uniform the toner layer on the surface thereof.
Especially, in relation to the developing device of the type in which the operation of developing is performed by causing the toner to fly without bringing the toner layer on the second developer carrying roller into contact with the surface of the latent image carrying member, it may be absolutely necessary to make the thickness of the toner layer on the second developer carrying roller. In other words, if the surface of the toner layer is not uniform, namely if it is in a state of undulation, surges or the like, the quality of the developed image may be apt to be deteriorated.